Engineers from MIT and Singapore University of Technology and Design (SUTD) have explored using light to 3D print complex structure that are able to spring back to their original forms within seconds of being heated to a particular temperature.

“We ultimately want to use body temperature as a trigger,” Fang says. “If we can design these polymers properly, we may be able to form a drug delivery device that will only release medicine at the sign of a fever.”

Shape-memory polymers can be used for a number of different applications including soft actuators that turn solar panels toward the sun, to tiny drug capsules that open upon early signs of infection, explains Fang.

The next step for the team is to explore 4D printing, as the structures are designed to change over the fourth dimension, time, explains Qi “Kevin” Ge, assistant professor at SUTD.

“Our method not only enables 4D printing at the micron-scale, but also suggests recipes to print shape-memory polymers that can be stretched 10 times larger than those printed by commercial 3D printers,” Ge says.

Shape-memory polymers can switch between two states: a harder, low-temperature, amorphous state, and a soft, high-temperature, rubbery state. The bent and stretched shapes can be “frozen” at room temperature, and when heated the materials will “remember” and snap back to their original sturdy form.

In order to print the new material, the team used a 3D printing process they have pioneered, called microstereolithography, in which they use light from a projector to print patterns on successive layers of resin.

The researchers used computer-aided design (CAD) software to create a model of the structure, then divide the model into hundreds of slices, each of which they send through the projector as a bitmap. The projector then shines light in the pattern of the bitmap, onto a liquid resin, or polymer solution, etching the pattern into the resin, which then solidifies.

The team picked two polymers to mix and create the ideal characteristics. One composed of long-chain polymers, or spaghetti-like strands, and the other resembling more of a stiff scaffold. When mixed together and cured, the material can be stretched and twisted dramatically without breaking.

The 3D printed structures could be stretched to three times their original length without breaking. When they were exposed to heat within the range of 40 C to 180 C, they snapped back to their original shapes within seconds.

“Because we’re using our own printers that offer much smaller pixel size, we’re seeing much faster response, on the order of seconds,” Fang says. “If we can push to even smaller dimensions, we may also be able to push their response time, to milliseconds.”

Going forward, Fang and the team hope to find combinations of polymers to make shape-memory materials that react to slightly lower temperatures, approaching the range of human body temperatures, to design soft, active, controllable drug delivery capsules. He says the material may also be printed as soft, responsive hinges to help solar panels track the sun.

Fang’s coauthors include former MIT-SUTD research fellow Qi “Kevin” Ge, now an assistant professor at SUTD; former MIT research associate Howon Lee, now an assistant professor at Rutgers University; and others from SUTD and Georgia Institute of Technology.

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